Mobile phones are designed to operate in multiple radiofrequency bands to be compatible with existing telecommunication standards (e.g., GSM, 3G, WCDMA, LTE or 4G). These standards may vary from country to country.
FIG. 1 shows the bands used by the 3G and LTE standards. The bands in white are those of the 3G standard, i.e., 824-960 MHz, 1710-1990 MHz, and 2110-2170 MHz. The shaded bands are those added by the LTE standard, i.e., 698-824 MHz, 1427-1496 MHz, and 2300-2690 MHz.
At these frequencies, especially in the 824-960 MHz band, the antennas have a relatively narrow useful bandwidth. The useful bandwidth is about 80 MHz, which causes difficulties in the design of broadband antennas.
FIG. 2A schematically shows a physical structure of an antenna that can cover all bands of the 3G standard. The antenna, called IFA (Inverted-F Antenna), is in the form of an “F” with two legs. One leg G forms the ground terminal and the other leg F forms the antenna's feed terminal. The two arms, which are of different lengths, are tuned on two conveniently chosen frequencies. Frequency f1 is for the longer arm and frequency f2 for the shorter arm.
FIG. 2B is a graph illustrating an exemplary graph of the reflection coefficient S11 of an IFA antenna as a function of frequency. The matching of the antenna is a maximum when the coefficient S11 is a minimum. It is considered that the matching of the antenna is sufficient when S11<−6 dB.
The frequency f1 is selected at the center of the 824-960 MHz band. The coefficient S11 exhibits a dip around this frequency, and remains below −6 dB over the major part of the band. The first harmonic 2f1 of frequency f1 happens to be at the beginning of the 1710-1990 MHz band, where the coefficient S11 has a new dip. The frequency f2 is selected so that the dip started at frequency 2f1 is maintained below −6 dB up to the end of the 2110-2170 MHz band.
To cover the missing bands in FIG. 1, one could consider adding properly sized arms to the IFA antenna of FIG. 2A. It turns out, however, that multiple-arm IFA antennas only operate properly if the gaps between the fundamental resonant frequencies are sufficiently large. As a result, IFA antennas do not have more than two arms. Another possibility is to use a parasitic grounded element to replace the second arm.
To cover all the bands, it has been proposed to use a tuning circuit that can modify the matching of an antenna to make it work over a larger number of frequency bands. This approach has the disadvantage of not changing the narrow-band nature of the antenna. A wider frequency band can thus be addressed, but all frequencies of the band may not be covered simultaneously.
In addition, the LTE standard provides, for increasing throughput, the ability to aggregate multiple paths that can be located anywhere in the standard bands. If the antenna tuning circuit technique were used in this situation, there would be a high likelihood that two aggregated paths be located in two bands not simultaneously covered by a same setting. As a result, one or more aggregated paths would be unusable.
Note that the low, 698-960 MHz band is particularly difficult to cover with a single antenna, since, as shown in FIG. 25, the antenna covers at most a band of about 80 MHz in this section. An IFA antenna could be provided, whose frequencies 2f1 and f2 are in the 698-960 MHz band, but the fundamental frequency f1, then on the order of 360 MHz, would require an oversized antenna arm and pose problems for integration into a mobile phone.
The article [Multi-Feed RF Front-Ends and Cellular Antennas For Next Generation Smartphones, Pekka Ikonen, Juha Ella Edgar Schmidhammer, Pasi Tikka, Prasadh Ramachandran, Petteri Annamaa], available on the website of Pulse Electronics, proposes an antenna circuit offering access to all standard bands through three separate feeds. Such an antenna circuit uses three independent RF processing paths, and specifically designed electronic circuits.